[Neuron] Add an option to build with neuron

[liangfu]

This PR adds an option that setup vLLM to build with Neuron toolchain (include neuronx-cc and transformers-neuronx).

This would help us build

vllm-0.2.3+neuron211

, where the neuron version comes out of the compiler version.

This is part of the effort to add support to accelerate LLM inference with Trainium/Inferentia (see #1866) .